The present invention relates in general to press brake machines and more particularly to a hydraulic control and actuating system therefor.
Press brake machines of the type in common use generally consist of a pair of spaced-apart upstanding frame members to the bottom ends of which a horizontal bed plate is mounted, and to the top ends of which a pair of hydraulic cylinders are mounted to reciprocatively drive a ram downward against the bed. The ram and bed are fitted with coacting dies and a work piece to be formed or sheared is placed between the dies. Upon actuation of a hand control switch or foot pedal by the operator, hydraulic power is supplied to the cylinders to bring the ram down with sufficient force to cause the dies to perform the desired forming or shearing action.
Where the length of the overall operating stroke of the ram is large relative to the working portion of the stroke, i.e. that portion of the stroke where pressure is actually applied to the work piece, provision is usually made in the hydraulic control system for advancing the ram initially at a relatively fast advance speed until the die nears contact with the work piece, and then for advancing the ram at a relatively slow pressing speed as the die actually engages the work piece and performs the desired forming or shearing operation. This results in a substantial savings in operator and machine time, without sacrificing the accuracy and safety achieved by the slow pressing speed.
An additional operational capability desirable in a ram control system is that it be capable of idling or holding the ram stationary at any selected position along its path of travel to permit the operator to reposition the work piece, or to set up the press brake for a subsequent operation. Furthermore, it is desirable that the system be capable of advancing the cam at its high or advance speed to a position adjacent the work, and holding it there until the operator actuates a hand switch or foot pedal to begin the actual pressing operation. Such high-stop-low (HSL) operation should preferably be entirely automatic and with a minimum amount of upward and downward creep of the ram. Since it is necessary for accurate bending and forming that the ram be brought squarely against the work piece with a minimum degree of tilt, i.e. variation from a desired angle of incidence, it is also desirable that the control system include tilt compensating means to adjust the hydraulic power applied to the cylinders to obtain an even descent.
To achieve the aforedescribed reciprocative operation of the ram, hydraulic press brake machines have been heretofore provided with a combined electrical and hydraulic control and actuator system which typically comprises multiple electrical limit switches disposed along the path of the ram to control its ascent and descent, and a differential valve responsive to the tilt of the ram to compensate for uneven descent. The limit switches, which are connected by appropriate electrical circuitry to solenoid-operated hydraulic valves, are mounted on the side of the press brake frame and operated by a cam actuator on the ram to control the application of hydraulic power to the cylinders. By adjusting the position of the switches the upward and downward travel limits of the ram can be preset, as well as the point at which the ram slows from its advance speed to its pressing speed. The differential valve, which is typically connected to the ram by a tape and pulley arrangement, compensates for uneven descent of the ram by reducing fluid flow to one or the other of the ram cylinders.
While such control and actuator systems have performed satisfactorily in larger press brakes, the hydraulic and electrical complexity of these systems has made them undesirably expensive for inclusion in lower capacity machines. Furthermore, the need for interlocking the hydraulic and electrical control circuits and the large number of solenoid-actuated valve required has precluded any significant degree of simplification or integration of the hydraulic and electric circuits of these systems. Thus, the need has developed for a less complex and more economical control and actuator system which provides the control functions found on larger and more expensive rams, including provision for advance and pressing speeds, HSL operation and ram alignment.